Engines may be configured with mechanical coolant pumps that circulate coolant through a cylinder head in addition to providing heat to a passenger compartment of a vehicle. Engine control systems may be configured to adjust the operation of the coolant pump based on engine temperature so as to maintain cylinder head temperatures. In some vehicle systems, such as hybrid electric vehicles, an electric auxiliary pump may also be included to assist the primary mechanical coolant pump.
One example of an engine system wherein an auxiliary pump is used with an engine-driven pump is disclosed by Aidnik in US 2004/0103862. Therein, a mechanical coolant pump provides coolant circulation during most driving conditions while an electric coolant pump is activated primarily during deceleration and after the engine is shut down. In particular, above a threshold engine speed, the mechanical pump is turned on while the electric coolant pump is cycled based on coolant temperature to provide auxiliary cooling control.
However, the inventors herein have identified potential issues with such a system. As one example, the amount of heat generated by the engine and/or the rate at which heat is generated at the engine may vary based on the nature of braking used during the deceleration conditions. For example, during conditions when the vehicle is decelerated using retarding forces within the engine (that is, compression braking), more heat may be generated at the engine as compared to conditions where the vehicle is decelerated using regenerative braking. As such, during compression braking, as the energy absorbed by the engine increases, more waste heat is generated that needs to be dissipated. Thus, if the coolant pump is operated based on the coolant temperature, as in the system of Aidnik, by the time the coolant pump is run, the coolant flow required to reduce the cylinder head temperatures may be substantially high. As such, this may increase the power consumption of the coolant pump. Further, even with the coolant pump operating at full flow at the time when coolant temperatures are high, cylinder head temperatures may not be reduced to desired levels in a suitable amount of time. As such, this may lead to localized boiling and coolant degradation.
In one example, some of the above issues may be addressed by a method of operating a hybrid vehicle system including an engine and a motor comprising, during conditions when the vehicle is propelled by the motor while the engine spins un-fueled, absorbing torque at the engine to maintain vehicle speed at a desired speed while adjusting operation of an engine coolant pump based on the absorbed torque to control engine temperature. In this way, coolant pump operation may be initiated and adjusted based on a compression braking torque to provide improved cooling control.
As an example, a hybrid vehicle system may include an electrically operated coolant pump. During conditions when the hybrid vehicle is propelled with only the motor, and the engine is spinning un-fueled while absorbing torque, such as during compression braking conditions, the coolant pump may be operated. In one example, pump operation may be initiated based on the vehicle speed relative to a radiator fan speed, so that coolant may start to be pumped through the engine as soon as compression braking starts. Then, as the amount of braking increases and the engine absorbs more torque, coolant pump operation (e.g., pump speed, pump flow, pump duty cycle, etc.) may be increased so that the pump operation can match engine heat generation.
In this way, by adjusting a coolant pump operation based on an engine compression braking torque, engine temperatures may be better controlled. Specifically, by increasing the amount of coolant flow with increased compression braking torque, it may be possible to better control the cylinder head temperatures while operating the coolant pump more efficiently.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.